1. Field of The Invention
This invention relates to x-ray imaging systems and particularly to a primary x-ray imaging system which utilizes a solid state x-ray detector to directly convert x-ray photons to usable electrical signals to form an image.
2. Discussion of Related Art
Since the discovery of x-rays and their first application in radiology for diagnostic imaging, some hundred years ago, the basic x-ray imaging systems and methodologies have stayed essentially unchanged. X-ray imaging involves transmission through the object by an x-ray beam and recording the image on photographic film producing a shadowgram. Computed Tomography (CT) is an exception and a special case because it produces a numerically computed image of a cross section of the object instead of a shadowgram.
Presently, x-ray imaging systems are utilized in a variety of applications, both as medical diagnostic tools and for industrial quality control. The most common form of x-ray detection resides in the use of silver halide film. However, the use of such film requires chemical developing steps. In addition, this film is expensive, thus increasing the cost of x-ray images produced in this manner.
For clinical diagnosis and routine screening of asymptomatic female population, screen-film mammography today still represents the state-of-the-art for early detection of breast cancer. However, screen-film mammography has limitations which reduce its effectiveness. Because of the extremely low differentiation in radiation absorption densities in the breast tissue, image contrast is inherently low. Film noise and scatter radiation further reduce contrast making detection of microcalcifications difficult in the displayed image. Film gradient must be balanced against the need for wider latitude.
Computed Radiography (CR) systems can be broadly categorized as primary digital and secondary digital systems. Primary digital systems imply direct conversion of the incident radiation on the sensor into usable electrical signals to form a digital image. Secondary digital systems, on the other hand, involve an intermediary step in the conversion of radiation to a digital image. For example, in digital fluoroscopy, image intensifiers are used for intermediary conversion of x-radiation to a visible image which is then converted to a digital image using a video camera. Similarly, digital x-ray systems using photostimulated luminescence (PSL) plates, first store the virtual image as chemical energy. In a second step, the stored chemical energy is converted into electrical signals using a laser to scan the PSL plate to form a digital image. Furthermore, various schemes using silicon photodiode arrays in scanning mode for CR systems have been employed. However, these photodiode arrays require intermediate phosphor screens to convert the x-radiation to visible light, because of the steep fall-off in quantum efficiency (sensitivity) of the arrays at energies above 10 KeV. The above described secondary digital systems have several disadvantages, including loss in image resolution.
Recent technological advances have made it possible to overcome these difficulties by allowing semiconductor x-ray detectors to be used to generate usable x-ray images. High quality semiconductor x-ray detectors have been known for many years, but these detectors require a very sensitive preamplifier to produce a useable signal. With recent advances in high density analog complementary metal oxide semiconductor (CMOS) integrated circuit technology and high density interconnection between semiconductor chips, the integration of thousands of these detector elements with preamplifiers on a single hybrid integrated circuit called a sensor chip is now possible. For example, Cox et al. U.S. Pat. No. 4,905,265, whose disclosure is incorporated herein by reference, discloses a semiconductor detector having an absorbing layer located between the x-rays from an object and the x-ray semiconductor sensors.